1. Technical Field
The invention relates to nuclear fuel assemblies and more particularly to grids for maintaining the fuel rods of a nuclear fuel assembly at a proper spacing, said grids including at least two spaced sets of plates, each set being formed of mutually parallel plates fixed at their ends to a polygonal belt in the form of a closed strip having fins along one edge at least.
Most light water cooled reactors now in operation use fuel assemblies of square cross-section, including a skeleton formed by two end pieces joined together by guide tubes and a plurality of grids welded to some at least of the guide tubes. The fuel rods are distributed in a square array and are substituted with guide tubes at certain of the nodal points of the array.
During loading into the core, or during removal of a fuel assembly, the latter is moved vertically with respect to the other assemblies. During this relative movement, it happens that a grid of the assembly being moved hooks onto a grid of an adjacent assembly, and hang up occurs. Such jamming generally occurs at a corner of a grid.
By way of example, FIG. 1 is a view from above showing how a grid 10 of an assembly being lowered into a reactor core may hook onto a grid 10a of an assembly already in the core. The guide fins 12 conventionally placed at the upper part of some at least of the grids prevent the grid of the assembly being lowered from sliding laterally if there is an overlap. Hang up may occur between two diagonally positioned assemblies, or between the flat sides of two grids of mutually adjacent assemblies, whatever the external shape of the assembly, between assemblies having a hexagonal cross-section as well as between those having a square cross-section.
2. Prior Art
Numerous attempts have been made to reduce the risk of hang-up which may irreparably damage the grids or even tear away sections thereof which are then driven into the primary coolant circuit. Arrangements for that purpose are described for instance in European Pat. No. 052,924 (Larson) and 141,208 (Steinke).
Arrangements have more particularly been proposed applicable to grids whose belt is formed by a strip having wide cuts defining bent tongues resiliently bearing on the rods. An approach described in French Pat. No. 2,337,918 consists in providing such grids with ribs projecting from the belt. This solution does not reduce the risk of hang-up at the corners. It has also been proposed, in grids whose belt has a height less than that of the plates, to bevel the end portions of the plates and to form a bevel on the corners of the belt (French Pat. No. 2,163,482). This construction has drawbacks and limitations pointed out in European Pat. No. 052,924 which suggest to shape the grid corners so as to define, at the top and bottom of each corner, an oblique gusset extending up to bends or folds formed parallel to the larger faces of the grids and which define tongues bearing on the rods and up to the ends of an opening formed in the corner of the belt and elongated in the direction of the rods.